Is the Blender GUI package available for other programs?
Short answer: no

...at least nobody has split off Blender's user interface from the application so that it can be used separately, and doing this isn't trivial.



Even if such a task were undertaken, keeping the code-bases in sync would be a challenge, so I think there is not much interest to do this. Besides, there are many UI toolkits out there.

There is GHOST (General Handy Operating System Toolkit), which can be used outside of Blender, but it's very low level, only providing window access, mouse, keyboard, ndof and tablet events from X11/MS-Windows/OS X.
It doesn't actually draw anything; buttons or text, for example, are handled by Blender's own code. So likely GHOST isn't what you want.

Rather then using GHOST you're better off using a general purpose library such as SDL, Allegro, GLFW, or SFML. Some of these libraries have Python wrappers; for example, PyGame is a popular SDL wrapper.

And, as for UI toolkits, there are too many to mention here; there are enough resources online.
See: Q What are the best cross platform widget toolkits?



On the other hand, nothing is stopping your from making a fork of Blenders UI for your own purposes, its just likely you'll end up having to spend time refactoring code out for your own use where you might better spend time working on the application its self.